Means to inject thermal energy into a workpiece for the nondestructive testing of that workpiece include lamp radiators as described in U.S. Pat. No. 3,427,861; hot-air guns as described in U.S. Pat. No. 3,378,685 or induction heaters as described in U.S. Pat. No. 4,215,562. Alternatively, the previously heated or naturally hot material of the workpiece may be surface cooled by a cool-air or water jet as described in U.S. Pat. No. 3,462,602. In this case, a thermal discontinuity over the defect will develop when the cold thermal front reaches subsurface flaws.
Methods to map the temperature distribution over the workpiece include single or double infrared sensors as described in U.S. Pat. Nos. 3,378,685 and 3,462,602,scanned detector images as described in U.S. Pat. No. 3,427,861 or thermosensitive coatings as described in U.S. Pat. No. 4,215,562. In modern thermal imaging systems, an infrared (IR) camera is used to continuously display the surface temperature of a thermally excited workpiece for inspecting relatively large areas at a fast pace.
Infrared inspection systems are difficult to apply to the nondestructive testing of materials having highly reflective surfaces, such as metallic sheets or aluminum-to-aluminum adhesively bonded panels of the kind used in the transportation and aeronautical industries. Metallic surfaces have a low light absorptivity and a low infrared emissivity coefficient, typically 5% of the emissivity of a black body. The infrared power radiated by a heated metallic workpiece is exceedingly low, giving a faint thermal image. In addition, the presence of grease patches or slightly oxidized areas on the metal surface may easily change the local surface emissivity from 5 to 10 or even 20% of that of the black body emissivity creating apparent "hot spots" on the infrared image which may be interpreted as defective areas. Moreover, the high reflectivity of the metallic surfaces introduces a reflection noise problem, whereby infrared radiation emitted by warm bodies in the background is reflected by a metal surface of the sheet which acts as a smooth mirror in the long-wavelength infrared region. The reflected image super-imposes itself onto the infrared emission image introducing further spurious "hot spots" and thus further complicating the interpretation of the thermal image provided by the infrared camera.
One possible approach to overcome these problems requires painting a black coating on the metallic surface prior to inspection. The black painted surface has a uniformly high infrared emissivity independently of the presence of grease patches, while its infrared reflectivity and thus the interference problems from nearby warm bodies is exceedingly low. However, the introduction of a painting step and of a subsequent cleaning step is quite inconvenient in an inspection procedure whose main interest is its high inspection speed.
Another possible approach to reduce low-emissivity problems is the use of a reflective cavity, such as the hemispheric cavity described in U.S. Pat. No. 4,480,168. Although being appropriate for the average temperature sensing of a given area, this approach is inadequate for thermal imaging because multiple reflections within the cavity tend to average out the infrared radiation emitted by the whole surface enclosed by the cavity.